178 Dr. A. Schrauf on certain Objections to the 



tion diminishes, while the other increases with elevation of tem- 

 perature, it follows that the influence of heat is considerably 

 influenced by the internal grouping of the atoms around the 

 axis. 



Although, however, this secondary influence of temperature 

 has long been known to me, I do not feel on this account in the 

 least compelled to regard the theory of refractive power as in- 

 correct. The following are, amongst others, my reasons : — 



In my " Studies" (p. 115) I have said, " Many hypotheses are 

 possible with regard to the variations of the magnitudes Z G X„. 

 The first is that the forces X w are dependent upon the chemical 

 properties. More attentive consideration, however, shows that 

 there can be no physical reason why a little mass of should 

 act differently upon the light to an equal mass of H. We are 

 hence led to the hypothesis that the retarding forces X„^are 

 equal for different elements — an assumption which is rendered 

 more than probable by the results obtained by it." 



This hypothesis may be retained for the first approximation, 

 which suffices in exactness for all hitherto performed investiga- 

 tions, even when we allow that X w is a function of the tempera- 

 ture. The abnormal variation of the refractive power M remains 

 within very narrow limits if all possible errors are eliminated. 

 It is further to be remarked that we are always at liberty, by 

 dint of comparing the refractive equivalents of various sub- 

 stances, to limit the constancy of X w to the ordinary tempera- 

 tures of observation. This I did in my " Studies." 



Such comparison of the retarding forces, X„, is possible under 

 all circumstances, and must take place under exactly the same 

 points of view under which the temperatures of the different 

 bodies are themselves measured. Even for the latter we are 

 accustomed to consider as equal the temperatures, and therefore, 

 implicitly, the molecular -path velocities of various bodies in dif- 

 ferent thermal states, although we know little about the law 

 of increase of temperature in bodies and the expenditure of free 

 heat necessary to effect it. 



In comparing the optical relations of various bodies, we can 

 therefore still use the theory of the refractive equivalent, inas- 

 much as for this theory the proportion 



X(«) 



X(b) 



is a constant for the same temperature. 



If, on the other hand, we compare the refractive indices of a 

 body for different temperatures, the twofold action of the influx 

 of conducted heat is clearly distinguishable. The overpowering 



